Tooth enamel has both organic and inorganic phases. The organic phase is composed of proteins, e.g., amelogenin, while the inorganic phase is composed of hydroxyapatite (Ca5(PO4)3(OH) or Ca10(PO4)6(OH)2, HAP) and substituted-hydroxyapatite (sHAP). The inorganic phase has ordered, crystalline phases of well-packed HAP crystals with some substitutions of the Ca, PO4, and OH groups with other molecules, such as other metals, fluoride, carbonates, hydrogen phosphates, and chloride. In biological systems, enamel can differ from pure HAP in stoichiometry, composition, crystallinity, and in other physical and mechanical properties. For example, biological apatites are usually calcium deficient and carbonate substituted. Thus, biological apatites can be referred to as carbonate apatite instead of hydroxyapatite (HAP). While, the composition of human enamel and of biological apatites are relatively known, the impact of trace elements on the physical-chemical properties, such as crystallite size, microstrain, hardness, and solubility of human enamel and sHAP is still of interest.
For example, it has been shown that some incorporated trace elements like Ti and Al are correlated with the mechanical and optical properties of naturally occurring human enamel. The incorporation of trace elements into human enamel can occur via biological processes; however, the concentration of these elements in human enamel—by as much as 1000× in some cases—is not well understood. As such, it would be useful to have methods to increase the concentration of certain trace elements in teeth to improve surface hardness, whiteness, and acid resistance of teeth. Compositions and methods for achieving these results have not been identified until now.
Over the course of a lifetime, teeth must resist daily physical insults including those from mechanical process that include chewing (attrition), brushing (abrasion), and bruxing (abfraction). The mechanical durability of a tooth is related to the surface hardness of the tooth, as well as its crack propagation resistance that are related to the trace element composition of the tooth. These properties can be influenced by modifying the chemical properties of human enamel. The benefit of such control would be increased durability of the tooth and longer lifetime of the tooth in situ. However, there have been few attempts to mitigate tooth loss from physical insult by changing the chemical structure of the tooth because the physical wear process, especially physical wear caused by cracking and fatigue failure from repeated loading, is poorly understood. Some processes by which physical insults can lead to mechanical wear include abrasion (loss via three-body wear), attrition (loss via grinding on occlusal surfaces), and abfraction (loss by repeated loading and cracking at the enamel/cementum interface). Tooth hardness and susceptibility to cracking are both influenced by the crystal size domain along the c-axis in human enamel. Several metal ions have been correlated with enamel c-axis crystal size, including, for example, Fe2+, Zn2+, Ti4+, Ce3+, and Al3+.
Teeth must also resist daily chemical insults, including multiple cycles per day to conditions where the tooth can be dissolved. In these circumstances, the aqueous environment local to the tooth is undersaturated relative to hydroxyapatite. A shift to undersaturated environment occurs when the pH is lowered from the biological homeostatic pH (roughly 6.5-8) to an acidic pH (roughly less than pH˜5 for typical biological levels of Ca and PO4 in saliva). pH in the oral cavity can be lowered by the metabolites of fermentable carbohydrate digestion by the oral cavity bacteria or by the consumption of low pH foods like wine, yogurt, or carbonated beverages. For example, fluoride substitution for hydroxide in human enamel can dramatically reduce the solubility of human enamel, because fluoroapatite (FAP or HAP with OH substituted with F) has a lower critical pH than HAP. Trace metals, when incorporated at the right degree of substitution, can slow the rate of enamel dissolution when exposed to acid. Both metals and fluoride, thusly, reduce the susceptibility of enamel towards dissolution. The chemical durability of a tooth, therefore, is related to the composition of the tooth near the surface. Consequently, the chemical durability, just like the mechanical durability, can be influenced by modifying the chemical properties of human enamel near its surface.
Typically, chemical damage to teeth is repaired through remineralizing without demineralizing teeth. The incorporation of elements to strengthen the tooth post-eruption rely on biological processes that first damage the tooth, which creates atomic vacancies in the apatite of enamel and dentin for the incorporation of fluoride and trace metals.
The additional incorporation of trace metals can further stabilize the apatite lattice of enamel by reducing the solubility of the tooth. Metals that can stabilize the apatites lattice of enamel include, for example, Mg2+, Sr2+, Sn2+, Ti4+, Al3+, Zn2+, Fe2+, Fe3+, Mo6+, B3+, Ba2+, and/or In3+. Additionally, trace metal content in drinking water is associated with a decreased caries rate. Thus, trace metal incorporation into the tooth can help slow acid damage.
Accordingly, there is a need for novel compositions and methods to chemically modify teeth to improve enamel hardness and increase the enamel's resistance to dissolution and acid erosion, without any damage to biological tissues that is typical during remineralization only processes. The present invention provides methods and compositions capable of exchanging ions with the hydroxyapatite mineral component of dental enamel, such that the resulting enamel is harder and more resistant to chemical and physical insult. Additionally, the present invention provides methods and compositions that can deposit precipitated coatings onto enamel that can be harder than the underlying surface. In this way, the intact tooth structure is altered prior to chemical or physical insult resulting in a tooth more resistant to damage. The present invention provides compositions and methods to demineralize and remineralize teeth to prevent damage to teeth caused by physical and chemical insults. The present invention provides compositions and methods to demineralize and remineralize teeth to prevent damage to teeth caused by physical and chemical insults.